1) Field of the Invention
The invention relates to a method of measuring at least one property of a moving web, in which method the property of the moving web is measured by means of a measuring device having at least one measuring head with a reference part, the reference part having a reference surface against which the moving web is supported to measure the property of the web.
Further, the invention relates to a measuring device for measuring at least one property of a moving web, the measuring device having at least one measuring head with a reference part, the reference part having a reference surface against which the moving web is arranged to be supported to measure the property of the web.
2) Description of Related Art
In paper manufacture, quality properties of paper, such as the thickness of the paper or another property, are typically measured in a continuous manner when a paper web is moving in a paper machine. Usually, measurement of the quality properties of paper is performed by what are called traversing measuring devices, in which the actual measuring detectors or sensors are arranged at measuring heads on the opposite sides of the paper web, the measuring heads, in turn, being arranged in measuring carriages traveling in the cross direction of the paper web over the whole width of the paper web. There is a narrow slot, i.e. an air gap, between the measuring carriages, where the paper web travels at high speed. The measuring devices can, however, be positioned fixedly, whereby they measure the same point in the cross direction of the paper web all the time.
In modern paper manufacture, there are very high requirements for measuring methods of the thickness of paper, in particular. For example, the thickness of the web manufactured with a newsprint machine is typically about 70 xcexcm, and the external measuring accuracy that is the aim of the measurement is below 1 xcexcm. The measuring accuracy to be aimed at may even be 0.3 xcexcm. Techniques used for measuring the thickness of paper are typically based on utilization of electromagnetism, optics or a combination thereof. In measuring devices utilizing optics, it is very important to keep the paper as straight and even as possible during the measurement; the aim is to guarantee this by bringing the web to the immediate vicinity of the measuring head, or even into contact with the measuring head, in which case the surface of the measuring head or a reference part arranged therein, being in contact with the web, forms the reference surface for the measurement. However, present solutions have the problem of the web vibrating at high frequencies when the paper moves forwards at a high speed, which vibration proceeds in the web to the measuring point of the thickness of the web, so that the web does not stay in contact with the reference surface at the measuring point, which leads to inaccurate measurement.
FI publication 973 448 discloses a measuring device based on electromagnetism, used for measurement of the thickness of a moving matter, such as a paper web, the device comprising a sliding panel against which the moving matter is supported for the measurement. There is/are one or more grooves machined as far as to the edge of the sliding panel in the direction of travel of the moving matter on the surface of the sliding panel. The grooves are machined in such a way that the width of the grooves increases as one moves towards the edge of the sliding panel. Further, there are channels machined on the sliding panel which supply pressurized air to the grooves from their narrower end, whereby negative pressure is generated between the sliding panel and the moving matter due to the effect of the pressurized air and the grooves, which negative pressure draws the moving matter against the sliding panel. The thickness of the moving matter can be determined by means of the distance between a measuring coil arranged on the sliding panel and a measuring probe arranged on the opposite side of the web relative to the measuring coil and the sliding panel. The problem in the solution disclosed in the publication is that when the moving matter is a thin paper web, for instance, the web easily vibrates when moving at a high speed, and blisters and bends due to the effect of the negative pressure in the grooves, whereby the web is easily lifted up from the reference surface formed by the sliding panel, and thus the measuring accuracy of the measuring device is weakened.
DE publication 19 913 928 discloses a measuring device for determining properties of a moving web and particularly the thickness of a moving paper web. The measuring device comprises an upper part arranged above the web, which upper part is kept at a distance from the upper surface of the web in such a way that it does not touch the web. Further, the measuring device comprises a lower part arranged below the web, which lower part is kept in contact with the lower surface of the web by means of the suction effect created by the design of the upper surface of the lower part and by the airflows caused by the air carried along the moving web, and possibly in addition, by means of special support means. The lower part of the measuring device can be brought in contact with the web also thanks to blow and suction effects directed at the web. According to an embodiment of the measuring device disclosed in the publication, the upper part of the measuring device comprises measuring means based on electromagnetism and optics for determining the distance between the upper and the lower parts of the measuring device and the distance between the upper part of the measuring device and the upper surface of the web and thus for determining the thickness of the web. In the solution disclosed in the publication, using air carried along the web to guarantee the contact between the lower part of the measuring device and the web is problematic, for example when the speed of travel of the moving web varies. Further, blisters may be formed in the moving web due to the design of the upper surface of the lower part of the measuring device and the effect of the negative pressure generated by the air carried along the web in such a way that the web can at some points be lifted up from the reference surface formed by the lower part of the measuring device, which weakens the measuring accuracy of the measuring device.
An object of the present invention is to provide a new type of solution to support a moving web against a reference part in a measuring device.
A method according to the invention is characterized by the reference surface comprising a measuring area, from the inside of which measuring area the property of the web is measured, and negative pressure being generated between the moving web and the reference surface through the reference part in such a way that the negative pressure affects the moving web in the direction of the web level at different distances from the measuring area in such a way that due to the effect of the negative pressure, the moving web is supported against the reference surface substantially over the whole area of the measuring area.
Further, the measuring device according to the invention is characterized in that the reference surface comprises a measuring area, from the inside of which measuring area the property of the web is arranged to be measured, and that the reference part is arranged in connection with the measuring head in such a way that below the reference part, there is a substantially open air space, and that the measuring head comprises means for generating negative pressure in the air space below the reference part, and that the reference part comprises holes formed through it at different distances from the measuring area in such a way that the negative pressure to be generated in the air space is arranged to affect, through the holes formed through the reference part, between the reference surface and the moving web in such a way that due to the effect of the negative pressure, the moving web is supported against the reference surface substantially over the whole area of the measuring area.
According to an essential idea of the invention, at least one property of the moving web is measured by means of a measuring device having at least one measuring head with a reference part, the reference part having a reference surface against which the moving web is supported to measure the property of the web. Further, according to an essential idea, the reference surface comprises a measuring area, from the inside of which measuring area the property of the web is measured, and that negative pressure is generated through the reference part between the moving web and the reference surface in such a way that due to the effect of the negative pressure, the moving web is supported against the reference surface substantially over the whole area of the measuring area. According to a preferred embodiment of the invention, the reference part is arranged in connection with the measuring head in such a way that below the reference part, there is a substantially open air space, and that the measuring head comprises means for generating negative pressure in the air space below the reference part, and that the reference part comprises holes formed through it, in such a way that the negative pressure to be generated in the air space is arranged to affect between the reference surface and the moving web through the holes formed through the reference part in such a way that due to the effect of the negative pressure, the moving web is supported against the reference surface substantially over the whole area of the measuring area. According to a second preferred embodiment of the invention, the measuring head comprises at least one channel that is in connection with the open air space below the reference part, whereby the negative pressure is generated in the air space by supplying a gaseous medium between the measuring head and the moving web. According to a third preferred embodiment of the invention, the lower surface of the reference part comprises two air channels substantially parallel relative to the direction of travel of the moving web for supplying a gaseous medium between the measuring head and the moving web substantially in parallel with the direction of travel of the moving web. According to a fourth preferred embodiment of the invention, the negative pressure between the reference surface and the moving web is arranged to be somewhat below the ambient pressure, typically 80 to 99% of the ambient pressure. According to a fifth preferred embodiment of the invention, the measuring device comprises a first measuring head and a second measuring head, between which there is an air gap where the moving web is arranged to travel, the reference part being arranged at the second measuring head. According to a sixth preferred embodiment of the invention, the first measuring head comprises both at least one electromagnetic measuring means for determining the distance between the first measuring head and the reference part at the second measuring head and at least one optical measuring means for determining the distance between the first measuring head and the web supported against the reference surface.
By means of the solution according to the invention, the moving web can be supported against the measuring area on the reference surface of the reference part in such a way that the web stays supported against the reference surface in the measuring area, despite vibrations generated while it moves, owing to which the property of the web can be measured very accurately. When the reference part is arranged in connection with the measuring head in such a way that there is a substantially open air gap below the reference part, and when holes extending through the reference part have been formed in the reference part, it is easy to generate negative pressure between the web and the reference surface by means of the negative pressure generated in the air gap. By supplying a gaseous medium between the web and the measuring head from a channel that is in connection with the air gap below the reference part, negative pressure can be generated in the air gap in a simple and easy manner. When a gaseous medium is supplied to the space between the measuring head and the moving web only in parallel with the direction of travel of the moving web, only a small amount of gas is required to provide sufficient negative pressure between the web and the reference surface. When the measuring device comprises a first measuring head and a second measuring head between which there is an air gap in which the moving web travels, and when the first measuring head comprises means for determining the distance between the first measuring head and the reference part arranged at the second reference head and means for determining the distance between the first measuring head and the web supported against the reference surface, it is easy to measure the thickness of the moving web at the accuracy of less than one micrometer.
In the present description, the term xe2x80x98paperxe2x80x99 refers, in addition to paper, also to board, tissue and pulp.